1. Field of the Invention
The present invention relates to a process for the preparation of glycosides. More particularly, it relates to a process for preparing glycosides efficiently from monosaccharides or polysaccharides without the need for using a large amount of acid catalysts.
2. Description of the Prior Art
Glycosides are very valuable as the starting material for glycoside polyethers which are used for the preparation of polyurethane foams (for instance, reference is made to U.S. Pat. No. 4,359,573, U.S. Pat. No. 4,366,265, U.S. Pat. No. 4,342,864, Modern Plastics International Aug. 1982, page 45, etc.). Glycoside compounds in which the saccharide portion is constituted by glucose are known in general as glucosides and they are useful as the starting material for cosmetics (for instance, see U.S. Pat. No. 4,323,468). Above all, methyl glucoside is particularly important (see Soap, Perfumery & Cosmetics, 47 (9), 406 (1974), U.S. Pat. No. 4,324,703, etc.). Methyl glucoside is also useful as a plasticizer for thermo-setting resins (see, Japanese Patent Publn. No. Sho 50-13770).
Up to the present, many processes for synthesizing such compounds have been reported. As representative examples, there can be mentioned.
(i) processes for the preparation of glycosides using hydrochloric acid as catalyst [Organic Synthesis, Collective Vol. (I) 364 (anhydroglucose is used as the starting material), Cantor et al. U.S. Pat. No. 2,390,507 (starch is used as the starting material). Roudier et al. FRP No. 1114382 (starch is used as the starting material)],
(ii) processes for the preparation of methyl glucoside using sulfuric acid as catalyst [Langlois U.S. Pat. No. 2,276,621 (starch is used as the starting material)],
(iii) processes using a Lewis acid as catalyst [Kaiser et al. U.S. Pat. No. 3,296,245 (starch is used as the starting material)],
(iv) processes for the preparation of glycosides using a cation exchange resin as catalyst [Cadotte et al. J. Amer. Chem. Soc., 74, 1501 (1952) (glucose is used as the starting material, and Amberlite IR-120 is used as the ion-exchange resin), Dean et al. U.S. Pat. No. 2,606,186 (glucose is used as the starting material, and Dowex 50, Duolite C-3, Amberlite IR-100, Zeo-Karb H or Nalcite HCR is used as the ion-exchange resin)],
(v) processes for the preparation of glycosides using a water-soluble sulfonic acid [Nevin et al. U.S. Pat. No. 3,375,243 (starch is used as the starting material), Roth et al. U.S. Pat. Nos. 4,223,129 and 4,329,449 (starch is used as the starting material, in both patents)], and so on.
Thus, many varied processes are known for the preparation of glycosides, and mineral acids are widely used as catalyst. Above all, hydrochloric acid is the most popular one. This is because the molecules of mineral acids, particularly hydrochloric acid, are small and accordingly the numbers of hydrogen ions per unit volume of acid are enormous. That is, they are very effective as acid catalyst. However, mineral acids are remarkably corrosive and require the use of acid proof apparatuses. In particular, highly volatile acids such as hydrochloric acid have a disadvantage in that they injure other apparatuses in the plant by gases escaping from them.
Such defect is common also to Lewis acids. Although reactions using a water-soluble sulfonic acid considerably ease the above-mentioned disadvantage, they have the disadvantage that an elevated temperature of 140.degree. C. or higher is required generally and the reaction mixture is considerably colored.
Moreover, in every case using one of the above-mentioned acids, the acid remains in the reaction system and so steps for neutralizing it and removing their salts are required.
To overcome these defects, processes using a cation exchange resin are recommended. However, cation exchange resins are generally inferior in their thermo-stability and accordingly it is impossible to effect reactions using them at an elevated temperature. The reactions are usually carried out at a lower temperature of 100.degree. C.-110.degree. C. and, as a matter of course, require a relatively long time (approximately 2 hours). Further, there is also a defect that the reaction system is deeply colored.